A modern tramper's pack consists of a sac in which equipment may be carried, the sac being mounted on a frame, and a shoulder harness secured to the sac or to the frame. Since carrying a heavy load supported only from the shoulders can lead to backache or even to back damage, a majority of modern packs also include a hip belt which is connected to the sac or the frame, generally at or close to the base of the sac, to transfer some of the load from the shoulders to the hips.
Although using a hip belt reduces the strain on the back, the belt itself can cause problems to the wearer, due to the way in which the human body moves when walking.
When a human being walks, he leans backwards and forwards with each step (FIG. 1) and from side to side with each double step (FIG. 2). As he leans to the supporting side when taking a step the torso compresses on that side and extends on the stepping side (FIG. 3): this means that, viewing the person from the front, the person's hips pivot up and down around an imaginary center line (indicated in broken lines in FIG. 3). This is termed `up and down` movement in the present specification. In addition, as shown in FIG. 4, the person's hips move with his legs, but his torso and arms swing in the opposite direction, with the body pivoting at the waist. In FIG. 4, a line through the shoulders is indicated by line S--S and a line through the hips is indicated by line H--H. As shown in FIG. 4, there is a considerable relative rotation between the shoulders (line S--S) and the hips (line H--H) with each step. This is termed `twisting` movement in the present specification.
Also, when a human being bends or steps up, his back extends:--the further the person leans forwards, the longer his back becomes. This extension of the back is in a curved plane, following the natural curve of the back.
Because of the above described movements, a hip belt which is rigidly secured to the pack, and which therefore moves with the pack, is very uncomfortable for the wearer, because the hip belt is constantly rubbing against the wearer due to the relative movements of different parts of the wearer's body when walking. The hip belt cannot be loosened to prevent this rubbing, because the belt must fit snugly to transmit load to the wearer's pelvic girdle and relieve the load on his back. FIG. 5 of the accompanying drawings shows, in diagrammatic form, the effect of a person wearing a pack with a rigidly-secured hip-belt bending forwards; the back extension raises the pack and lifts the hip-belt from the hips, applying uncomfortable pressure across the front of the lower abdomen (arrow P).
The above described problem was partially solved by the pivotal hip belt connection provided by New Zealand patent No. 201751 (Macpac Wilderness Equipment Limited) dated Jul. 25, 1983.
However, the hip belt described in New Zealand patent No. 201751 was not a complete solution to the problem:--because the hip belt was secured to the pack by a comparatively narrow connection, the load transfer from the pack to the hip belt was concentrated at the connection point and was distributed around the pelvic girdle of the wearer only by the stiffness of the hip belt. The more rigid the hip belt, the better the load transfer, but in general, the more rigid a hip belt is, the less comfortable it is to wear. Thus, the problem remained of providing a harness which permitted relative movement between the wearer's back and hips in at least three directions but which provided an efficient load transfer around the pelvic girdle of the wearer without resorting to the use of an uncomfortably stiff hip belt.
There have been a number of other attempts to solve this problem: for example, U.S. Pat. No. 5,503,314 dated Apr. 2, 1996 proposes a back pack carrier incorporating a crisscross framework which is designed to have inherent flexibility because of its sinusoidal shape and which also is made of flexible material. The carrier also incorporates a separate shock absorbing mechanism. The base of the crisscross framework is rigidly secured to a hip belt; thus, any flexing of the hip belt relative to the pack is governed by the flexibility of the crisscross framework.
International patent application PCT/US97/11152 discloses a load support system which consists of a flexible frame unit made of a shaped plate of flexible material formed with lugs or out riggers which extend around the sides of the wearer and are secured to pivots attached to the sides of a hip belt. This gives a pivotal connection between the frame unit and the hip belt, but only in a vertical plane: any twisting of the wearer's hips relative to the wearer's shoulders has to be accommodated by the flexibility of the frame unit itself.
International patent application PCT/US97/13396 discloses an internal frame back pack in which the frame is formed by a shaped sheet of resilient material reinforced with resilient rods. The lower edges of the backpack are rigidly secured to a hip belt, so that any twisting of the wearer's hips relative to the wearer's shoulders again has to be accommodated by the flexibility of the frame.
In all three of the above inventions, the frame is described as "flexible". However, a backpack frame cannot be extremely flexible, or it simply will not support the load in the pack. Further, no matter how flexible a frame is in itself, it will lose much of its flexibility when the pack is loaded, since the presence of a full, heavy, essentially rigid, loaded pack secured to the frame makes it difficult for the frame to flex very much.
It is therefore an object of the present invention to overcome the above described problems by providing a pack in which part of the load is transferred efficiently from the shoulder harness to the pelvic girdle of the wearer via the hip belt, without sacrificing a flexible connection between the sac and the hip belt to allow comfortable movement as described above, and without resorting to an uncomfortably stiff hip belt.